It is becoming increasingly common to include security systems on vehicles which enable door locks and engine immobilisation to be operated remotely by a portable transmitter unit. It is also known to provide a `lazy lock` function as part of such systems in which electrically powered windows and sunroof of the vehicle are closed in response to a signal from the transmitter unit requesting locking of the vehicle. However it is a problem with such systems that, whilst it is desirable for the doors to be lockable from a relatively large distance, operation the lazy lock function from such a distance can be dangerous because of the risk of somebody becoming trapped by the closure whilst out of sight of the person operating the transmitter unit.
In U.S. Pat. No. 5,600,323 a system is disclosed which aims to address the problem of undesirable locking or unlocking of a vehicle at distances which may be out of sight of that vehicle. This system divides the area around the vehicle into zones and enables or disables preselected systems in accordance with the presence or absence of the transmitter unit in an associated zone.
It is a problem with the known closure system that it relies on detecting the level of the transmitted signal in order to determine in which zone the transmitter is being operated. The signal may be susceptible to interference from another source or from its surroundings and give rise to incorrect enablement or disablement of a subsystem as a result of a false determination of signal strength.